The long term goal of the proposed research is a better understanding of the molecular basis of vertebrate development. The research proposed here will focus on one of earliest events of vertebrate development, gastrulation. During gastrulation there is both extensive reorganization of embryo, initiating the sequence of events that give vertebrates their unique morphology, and an extensive period of organogenesis, giving rise to many of the major organs an tissues, including the central nervous system and skeletal muscle. The embryos from the toad Xenopus laevis have proven to be very useful in studying early development, and much that has been learned with Xenopus has been found to be applicable to higher vertebrates, including mammals. The organogenesis and morphogenesis of the gastrulating Xenopus embryo is regulated by a small region of the dorsal mesoderm, the Spemann organizer. The Spemann organizer is able to direct development at the gastrula stage by secretion of soluble inductive factors. An expression cloning approach has been used to identify factors produced by the organizer. One of the factors isolated in this screen, dor3, is a novel member of the TGFbeta family of growth factors. The function of dor3 int he gastrulating embryo will be investigated in several ways. Preliminary evidence suggests that dor3 works in coordination with other organizer-specific growth factors, noggin in particular, to induce the cell movements of gastrulation. Experiments will look at both the effects of adding exogenous dor3 to gastrulating embryos, and at the effects of neutralizing endogenous dor3, with use of dominant negative mutants. Recombinant dor3 protein will be used to isolate the dor3 receptor, a first st ep in making a dominant negative receptor. Dor3 appears to be expressed in a subregion of the organizer. An extensive series of in situ hybridizations will be done with dor3, and other organizer specific genes, to look at structure of the organizer. The set of tissue inductions and morphogenetic movements that occur in the gastrulating Xenopus embryo represents a very accessible, and relatively simple, model of organogenesis. Through investigation of the interactions of growth factors in the gastrulating embryo, insights is gained into the general phenomenon of tissue and organ development. This information should ultimately prove useful to the goal of repair and regeneration of human tissue damaged by accident or disease.